Node.js - Handle body-parser invalid JSON error

Question

I'm using the body-parser package like this:

// For parsing application/json:
app.use(require('body-parser').json());

// For parsing application/x-www-form-urlencoded
app.use(require('body-parser').urlencoded({ extended: true })); 

When a valid input like { "foo": "bar" } is received everything works fine and I can access the parsed object with req.body.

However, when invalid (non-JSON) data is sent:

data: JSON.stringify("just something inappropriate"),

I get this error:

{ SyntaxError: Unexpected token " in JSON at position 0
    at JSON.parse (<anonymous>)
    at createStrictSyntaxError
    at ...
expose: true,
statusCode: 400,
status: 400,
body: '"Something"',
type: 'entity.parse.failed' }

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client at ...

How can I handle this properly to prevent the server from shutting down?

Answer 1

One option is to add a custom error handler middleware and add a check to catch JSON parsing errors like that one:

app.use(require('body-parser').json()); 
app.use(require('body-parser').urlencoded({ extended: true }));

...

app.use((err, req, res, next) => {
    // This check makes sure this is a JSON parsing issue, but it might be
    // coming from any middleware, not just body-parser:

    if (err instanceof SyntaxError && err.status === 400 && 'body' in err) {
        console.error(err);
        return res.sendStatus(400); // Bad request
    }

    next();
});

Another option is to wrap body-parser's middleware to catch errors coming only from there:

const bodyParser = require('body-parser');

app.use((req, res, next) => {
    bodyParser.json()(req, res, err => {
        if (err) {
            console.error(err);
            return res.sendStatus(400); // Bad request
        }

        next();
    });
});

Or if you want to reuse this functionality to catch different errors from different middlewares, you can do:

function handleError(middleware, errorHandler) {
    middleware(req, res, err => err ? errorHandler(err, req, res, next) : next());
}

const bodyParser = require('body-parser');

app.use(handleError(bodyParser.json(), (err, req, res, next) => {
    if (err) {
        console.error(err);
        return res.sendStatus(400); // Bad request
    }

    next();
}));

Answer 2

As of Express 4.16.0 - Release date: 2017-09-28 you can catch different errors from different middlewares splitting the error handler into a different function without using Bodyparser as it is deprecated.

const app = express();

function handleError(middleware, req, res, next) {
  middleware(req, res, (err) => {
    if (err) {
      console.error(err);
      return res.sendStatus(400); // Bad request
    }

    next();
  });
}

app.use((req, res, next) => {
  handleError(express.json(), req, res, next);
});

Notice in the code as of Express 4.16.0 and higher that express:

 app.use(express.json()); // Without middleware error handling.

replaces bodyParser:

 app.use(bodyParser.json()); // Without middleware error handling.

Answer 3

Add an error handler and then customize the behavior of the default way to handle that erorr, the default will be to crash as you describe.

app.use((err, req, res, callback) => {
  // todo: implement error handling logic and return an appropriate response
  console.error(err)
  res.send(500)
  callback()
})